Olefins, such as ethylene and propylene, have become major feedstocks in the organic chemical and petrochemical industries. Of these, ethylene is by far the most important chemical feedstock since the requirements for ethylene feedstocks are about double those for propylene feedstocks. Consequently, improved methods for the conversion of less valuable hydrocarbons to ethylene and propylene, and particularly to ethylene, are highly desirable.
Numerous suggestions have been made for the production of ethylene and propylene, particularly ethylene, from various feedstocks by a wide variety of processes.
At the present time, ethylene is produced almost exclusively by dehydrogenation or thermal cracking of ethane and propane, naptha and, in some instances, gas oils. About 75% of the ethylene currently produced in the United States is produced by steam cracking of higher normally gaseous hydrocarbon components of natural gas, since natural gas contains from about 5 volume % to about 60 volume % of hydrocarbons other than methane. However, in most instances, the content of ethane and higher normally gaseous hydrocarbons in natural gas is less than about 25% and usually less than about 15%. Consequently, these limited quantities of feedstocks, which are available for the production of ethylene and propylene and particularly ethylene, must be utilized efficiently. Unfortunately, these processes result in low conversions to olefins, and selectivity to ethylene, as opposed to propylene, is poor. In addition, relatively severe conditions, particularly temperatures in excess of about 1000.degree. C., are required and such processes are highly energy intensive.
In order to reduce the severity of the conditions and, more importantly, to improve the conversion of normally gaseous feedstocks, to ethylene and propylene, and the selectivity to ethylene, numerous processes involving the use of solid contact materials have been proposed. Some of these proposals utilize inert solid contact materials to improve contact between the feed hydrocarbons and steam and also to maintain a more even temperature throughout the zone of reaction. In other instances, the solid contact material is catalytic in nature. Such use of solid contact materials, particularly catalysts, have resulted in modest improvements in conversion to ethylene and propylene but the selectivity to ethylene is improved very little. It is, therefore, highly desirable that improved catalytic processes be developed, particularly processes which increase the selectivity to ethylene, as opposed to propylene. However, little is understood concerning the manner in which such catalysts function, why certain components are effective while similar components are ineffective or why certain combinations of components are effective and other combinations are not. Obviously, a number of theories have been proposed by workers in the art, but this only adds to the confusion, since it appears that each theory explains why a particular catalytic material works well but does not explain why similar catalytic materials do not work and why other dissimilar materials are effective. As a result, the art of catalytic conversion of hydrocarbons to olefins remains highly unpredictable.